1. Field of the Invention
The present invention relates to a fixing unit with a cooling section for an eleotrophotographic apparatus, and an image forming apparatus with a cooling section.
2. Description of the Related Art
An image forming apparatus, such as an electrophotographic apparatus, includes a fixing unit for fixing the toner image onto a print medium. The fixing roller of the fixing unit is heated up to a high temperature and the surface temperature is controlled to stay within a predetermined range.
FIG. 8 shows a temperature control circuit of a conventional fixing unit. The conventional fixing unit includes a heating section 1 for heating a fixing roller 10, a cooling section 2 for blowing air into the interior of the unit to cool the fixing roller 10, a temperature detection section 3 for detecting the surface temperature of the fixing roller 10, and a control section 4 for sending a control signal to the heating section 1 to heat the fixing roller if the detected temperature of the fixing roller 10 is below a predetermined temperature or to stop heating the fixing roller 10 if the detected temperature is above the predetermined temperature.
The heating section 1 includes a series circuit of a heater 1-1 for heating the fixing roller 10, a thermostat 1-2 for preventing overheat of the fixing roller 10 due to a malfunction, an a-c current source 1-3 for supplying the heater 1-1 with heating current, and a switching circuit 1-4 for turning on/off the a-c current source 1-3 based on control of the control section 4. An amplifier consisting of a transistor Tr1, a resistor R3, and a resistor R8 to receive and amplify a control signal from the control section 4 is connected to a control terminal of the switching circuit 1-4.
The cooling section 2 includes a fan 2-1 for cooling the unit interior, an amplifier consisting of a transistor Tr4, a resistor R9, and a resistor R10 for amplifying a control signal from the control section 4, and a driving amplifier consisting of a transistor Tr5 and a resistor 11 for driving the fan 2-1. The temperature detection section 3 includes a thermistor 3-1 for detecting the temperature of a fixing roller 10 and a voltage dividing resistor R1. The voltage at a connection point between the thermistor 3-1 and the voltage dividing resistor R1 is sent to a port P1 of the control section 4 as the detected temperature.
The control section 4 is made of a central processing unit (CPU) and connected to a read only memory (ROM) and a random access memory (RAM). According to a readable program stored in the ROM or RAM, it receives the detected temperature from the temperature detection section 3 and executes a predetermined process, sending a control signal to the heating section 1.
In the above conventional fixing unit, if the control section 4 becomes out of control or the switching circuit 1-4 is down, an unexpected heating current can be continuously supplied to the heater 1-1 to raise the temperature of the fixing roller above the necessary level. If the fan 2-1 keeps rotating under this condition, the raised temperature spreads over the entire equipment, preventing normal printing. There is a technique that the sensitivity of the thermostat 1-2 is enhanced to prevent the extreme increase in the temperature of a fixing roller.
In the conventional fixing unit, however, it is impossible to prevent the raised temperature from spreading over the entire unit, causing a malfunction. In addition, there is a limit to the improvement in the thermostat and it is difficult to completely prevent the continuous supply of unexpected current to the heater 1-1.